This invention relates to temperature maintaining apparatuses such as transit vehicle heaters and temperature control apparatuses and methods therefor.
Various temperature maintaining apparatuses, particularly heaters, operate in a cyclic fashion. When the temperature reaches a certain threshold, such apparatuses operate for a period of time until the temperature is brought to a certain level and thereafter the apparatus becomes nonoperational until the threshold is reached again. For example, in a heater the temperature may be the temperature of a fluid and the heater operates when the temperature of the fluid drops to a set lower value. The heater continues to operate until the temperature is raised to a higher level at which time the heater switches off until the temperature again drops to the lower level.
One specific type of heater is a transit vehicle heater which operates to heat the engine coolant. The temperature of the coolant is sensed by at least one temperature sensor and the heater, typically operating on diesel fuel, is fired until the temperature of the coolant is raised to a higher level. At this point the heater switches off.
This cycling on and off between preset lower and higher temperatures produces less than satisfactory performance under some conditions. People in a transit vehicle, for example, may perceive that the temperature of the air in the vehicle drops to an uncomfortable level when the heater cycles on and off between preset lower and upper temperatures. This is because too much time may occur near the bottom of the cycle when the temperature is low and/or because the mean temperature is reduced below optimum levels. Standard on/off temperature controls or thermostats are not capable of adjusting themselves to such a situation.
Transit vehicle heaters, as well as some other types of heaters for liquids, conventionally employ two different temperature sensors. One of the sensors contacts the liquid, typically a water/anti-freeze mixture in the case of a transit vehicle heater, to determine the temperature of the liquid in order to cycle the heater on and off as required to maintain the temperature of the liquid. A second sensor contacts the inner wall of the jacket to sense an overheat condition. The need for two different sensors increases the cost and complexity of the units.
Transit vehicle heaters, and other types of heaters often must be installed in different configurations. For example, in the case of transit vehicle heaters, the direction of flow of liquid through the heaters sometimes must be reversed according to the plumbing requirements of a particular vehicle. Conventional transit vehicle heaters do not readily adapt to a reversal of liquid flow because of temperature sensors placed adjacent the fluid inlet and fluid outlet. The heater controls are adversely affected when the fluid flow is reversed because of the effect this has on the readings of the sensors.
Also some conventional heaters require two sensors adjacent the fluid inlet and fluid output respectively and cannot operate with a single sensor or with one of the sensors disabled. The controls have a lack of flexibility in this respect.
It is an object of the invention to provide an improved heat transfer device, such as a transit vehicle heater, as well as controls therefor and a method of maintaining a temperature, which are capable of adapting to different temperature conditions to maintain a more constant mean temperature compared to a simple system where the device cycles on and off at fixed temperatures.
It is another object of the invention to provide an improved heater for liquids, a method of controlling temperatures for liquids and a temperature control apparatus where a single sensor can act both to measure temperatures of the liquids to determine when the heater cycles on and off, and also acts as an overheat sensor for the heater.
It is a further object of the invention to provide an improved heater for liquids, a method of controlling such heaters and control systems for such heaters which are adaptable to varying installation requirements, such as reversing the flow of liquids through the heaters.
It is still further object of the invention to provide an improved heater for liquids, a method of controlling such heaters and a control systems for such heaters which can operate with two temperature sensors, with one of the sensors removed or with one of the sensors disabled.